The present invention is directed to a variable density light transmitting device and, more particularly, to a variable density light transmitting device comprising at least two layers wherein density variation is achieved through linear shifting of one layer relative the other layer.
The optical phenomenon known as polarization depends on the fact that each light wave vibrates in a plane which extends transversely to the direction of wave propagation. A beam of natural light contains a mixture of light waves vibrating in a plurality of different transverse planes oriented in a full 360.degree. about an axis defining the direction of wave propagation. Certain materials exhibit a property known as "dichroism" meaning the property of differential absorption of the component waves of an incident beam of light depending upon the vibration direction of the component waves. A polarizer formed from a dichroic material may be used to selectively allow transmission of light waves vibrating in a direction parallel to the transmission axes of the polarizer while preventing transmission of light waves oriented in a direction perpendicular to the transmission axes. An early commercially viable form of polarizer invented by Edwin H. Land in 1928, consists of a thin layer of tiny needle-like dichroic crystals of herapathite, in parallel orientation, embedded in a plastic matrix and enclosed for protection between two transparent plates. A modification, developed by Land in 1938 and known as H-Sheet, is a molecular polarizer. It consists of long polymeric molecules of polyvinyl alcohol (PVA) that has been given a preferred direction by stretching and have been stained with an ink containing iodine that causes the sheet to exhibit dichroism. The PVA sheet is laminated to a support sheet of cellulose acetate butyrate. The existence of relatively low cost polaroid sheets formed by the process developed by Land and other similar processes, has resulted in applications for polarizers in numerous different optical apparatus. Two of the more common uses are for sunglass lenses and defraction gradiants for camera lenses. Due to the fact that light transmitted through a polarizing layer is linearly oriented or "polarized", it is possible to change the amount of transmission of light through a variable density device consisting of two polarizing layers by rotation of one polarizing layer which respect to the other polarizing layer. In a light beam passing through a first polarizing layer and then through a second polarizing layer, the amount of light transmitted through the second polarizing layer may vary from a maximum amount of light equal to that transmitted through the first polarizing layer when the transmission axes of both layers are parallel to theoretically no light transmission whatsoever, i.e. total extinction, when the transmission axes of the two polarizing layers are positioned perpendicular to one another. The mathematically expression for this relationship was discovered, experimentally, by Etienne Louis Malus in 1809, and is known as Malus Law. Malus Law is expressed as I=I.sub.max COS.sup.2 Q, where I is the amount of light transmitted through the second layer, I.sub.max is the amount of light transmitted through the first layer, and Q is the angle between the transmission axis of the first layer and the transmission axis of the second layer. The advantage of variable density light transmission for sunglasses, through the use of two polarized layers which are rotatable with respect to one another, was recognized by E. H. Land in U.S. Pat. No. 2,005,426, the disclosure of which is hereby incorporated by reference. A number of other adjustable sunglasses wherein the light density is adjusted by rotation of one polarizing layer with respect to another polarizing layer, are described in U.S. Pat. Nos. 2,298,058; 2,773,422; 3,423,149; 3,944,346; 4,264,154; and 4,386,832 which are all hereby incorporated by reference. These subsequent patents deal with various mechanisms for rotating one eyeglass lens with respect to another eyeglass lens and, in some cases, deal with the use of differential color absortive or differential color polarizing lenses.
A problem with all such prior art sunglasses and with such means of varying light density in general, is that the shape of the light transmissive layers must generally be limited to a circular or near-circular shape in the area where variable density is to be achieved. Such circular shapes are generally not desirable in all sunglasses and in other potential applications for a variable density light transmission device such as, for example, windows. Another problem is that a relatively large angle deflection of one layer relative to the other layer is necessary to achieve a significant change in light transmission. The human eye perceives light logarithmically and thus even a 50% transmission reduction produced by 45.degree. rotation of one layer relative the other layer may seem to be a relatively small change in light transmission to the users of such glasses. A problem associated with the large angle deflections in the case of sunglasses is that a lens portion for each eye must be rotated by the same amount simultaneously. Mechanism for producing such coordinated large angle deflections in two separate lenses have been either awkward or expensive and have limited the practical application of such prior art variable density sunglasses.
It would be generally desirable to provide a device capable of achieving the variable density light transmission described in the above cited patents without requiring circular lens portions or cumbersome large angular displacements of one polarizing layer relative to another polarizing layer.